This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-016491, filed Jan. 26, 2000; No. 2000-016492, filed Jan. 26, 2000; and No. 2000-372946, filed Dec. 7, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a flexible wiring board, a method of manufacturing a flexible wiring board, and a display device such as a liquid crystal panel for a liquid crystal display module connected to a flexible wiring board.
FIGS. 12 to 14 collectively show an example of a conventional liquid crystal display module. As shown in the figures, the conventional liquid crystal display module comprises a liquid crystal display panel 1 and a flexible wiring board 11. The liquid crystal panel 1 is prepared by bonding a segment substrate 2 and a common substrate 3 to each other with a substantially frame-like seal member 5 interposed therebetween and by sealing a liquid crystal 7 between these substrates 2 and 3 positioned inside the seal member 5. Segment electrodes 2a and a common electrode 3a are mounted to those surfaces of the substrates 2 and 3, respectively, which are positioned to face each other so as to permit a predetermined voltage to be applied to the liquid crystal 7. The lower side portion in FIG. 12 of the segment substrate 2 protrudes from the lower side of the common substrate 3 to form a protruding portion 2b. A plurality of connection terminals 4 including segment terminals connected to the segment electrodes 2a and common terminals connected to the common electrode 3a via the seal member 5 are arranged in parallel on one surface of the protruding portion 2b. 
The flexible wiring board 11 comprises an oblong film substrate 12 on which electronic parts are mounted by a TAB (tape automated bonding) system. The film substrate 12, which is about 75 xcexcm to 150 xcexcm thick, is formed of, for example, a polyimide film. A semiconductor chip 13 for driving the liquid crystal display panel 1 such as an LSI and chip parts 14 required for driving the liquid crystal display panel 1 and each comprising a capacitor, a resistor, etc. are mounted on predetermined positions in substantially the central portion of the film substrate 12. In this case, the semiconductor chip 13 is mounted on a region the film substrate 12, in which a device hole 15 is formed.
A large number of output wirings 16 are connected to the semiconductor chip 13 on the upper surface of the film substrate 12 in an upper region in FIG. 12 of the mounting region of the semiconductor chip 13, and a large number of input wirings 17 extending in parallel are connected to the semiconductor chip 13 and the chip parts 14 in the lower portion in FIG. 12 of the mounting region of the semiconductor chip 13. The lower end portion in FIG. 12 of each input wiring 17 constitutes a first connection terminal 17a, with that portion of each input wiring 17 which protrudes into the device hole 15 constituting a second connection terminal 17b. 
The output wiring 16 will now be described. It should be noted that two slits 18 and 19 extending in parallel in a lateral direction are formed in the film substrate 12 in predetermined two positions in an upper portion in FIG. 12 of the mounting region of the semiconductor chip 13. The roles played by these two slits 18, 19 will be described herein later. The output wirings 16 comprise a plurality of first connection terminals 16a formed in parallel in an upper end portion of the film substrate 12, a plurality of second connection terminals 16b protruding into the device hole 15 in a portion of the device hole 15, first drawing wire sections 16c formed in parallel in the portions of these slits 18, 19 and between these slits 18 and 19, and second drawing wire sections 16d arranged between the first drawing wire sections 16c and the second connection terminals 16b such that the pitch of these second drawing wire sections 16d is gradually increased from the second connection terminals 16b toward the first drawing wire sections 16c. A protective film 10 made of a solder resist is formed on the upper surface of the film substrate 12 including the wirings 16 and 17 except the mounting region of the semiconductor chip 13, the mounting region of the chip parts 14, the upper end portion in FIG. 12 including the first connection terminals 16a and the lower end portion including the second connection terminals 17a. The protective film 10 is provided with slits 10a and 10b in positions corresponding to the slits 18 and 19, respectively, of the film substrate 12.
The bonding portions comprising the portions of the first connection terminals 16a of the flexible substrate 11 are bonded to the bonding portions comprising the connection terminals 4 of the liquid crystal display panel 1 with an anisotropic conductive adhesive (not shown) interposed therebetween.
The reason for forming the device hole 15 in the film substrate 12 will now be described. First the mounting state of the semiconductor 13 will be described. A plating layer (not shown) made of a metal having a low melting point such as tin or a solder is formed on the surfaces of the output wirings 16 and the input wirings 17. As shown in FIG. 13, the upper surfaces of a plurality of bump electrodes 6 made of gold and formed in a peripheral portion on one surface of the semiconductor chip 13 are bonded to the upper surfaces of the second connection terminals 16b, 17b by a eutectic alloy of gold-tin or gold-solder so as to bond the semiconductor chip 13 to the portion surrounding the device hole 15 of the film substrate 12.
When the bump electrodes 6 of the semiconductor chip 13 are bonded to the connection terminals 16b, 17b by a eutectic alloy, the semiconductor chip 13 is disposed on a stage (not shown), and then the film substrate 12 is moved to a region above the semiconductor chip 13 so as to align the positions of the connection terminals 16b, 17b of the film substrate 12 with the positions of the bump electrodes 6 of the semiconductor chip 13. Further, a bonding tool is brought into direct contact with the connection terminals 16b, 17b for the pressurizing under heat. For the operation described above, the device hole 15 is formed in the film substrate 12. The reason for forming the device hole 15 is as follows. Since the film substrate 12 is relatively thick, i.e., the thickness is about 75 xcexcm to 150 xcexcm, the film substrate 12 is unfavorably melted before the bump electrodes 6 and the connecting terminals 16b, 17b are heated to reach the bonding temperature, when the film substrate 12 is pressurized directly from above by the bonding tool at 530 to 550xc2x0 C. without forming the device hole 15. As a result, a defective bonding is generated by the deviation in the position of the wiring.
An example of mounting the liquid crystal display module shown in FIG. 12 to a circuit board will now be described with reference to FIG. 14. The liquid crystal display panel 1 is disposed in a predetermined position on the upper surface of a circuit board 21 with the segment substrate 2 being positioned on the lower side. The flexible wiring board 11 is bent at substantially 90xc2x0 in each of the portions of the slit 18(10a) and 19(10b). The portion between the slits 18 and 19 of the flexible wiring board 11 is inserted into a slit 22 formed in a predetermined position of the circuit board 21, and the portion below the circuit board 21 is allowed to extend along the lower surface of the circuit board 21. Under this state, the bonding portions comprising the second connection terminals 17a of the flexible wiring board 11 are bonded to bonding portions comprising the connection terminals formed at predetermined positions on the lower surface of the circuit board 21 with an anisotropic conductive adhesive (not shown) interposed therebetween so as to communicate with an electronic part 9 via a wiring 8.
The roles played by the slits 18(10a) and 19(10b) will now be described. It should be noted that the film substrate 12 of the conventional flexible wiring board 11 is relatively thick, i.e., about 75 xcexcm thick. Also, the flexible wiring board 11 is bent when the display module is housed in a package. If the slits 18 and 19 are not formed, it is difficult to bend the flexible wiring board 11 as desired. Therefore, the slits 18 and 19 are formed so as to facilitate the bending of the flexible wiring board 11. Also, if the slit is formed in the arranging region of the second drawing wiring sections 16d, the second drawing wiring sections 16d are put in an inclined state toward the longitudinal direction, i.e., in the vertical direction in FIG. 12, of the film substrate 12. As a result, the length of each second drawing wiring section 16d corresponding to the slit is made larger than the length in lateral direction of the slit. In addition, the presence of the slit causes the second drawing wiring section 16d not to be supported by the film substrate 12 and, thus, to be irregularly twisted, giving rise to the occurrence of a short circuit. Such being the situation, a slit is not formed in the arranging region of the inclined second drawing wiring sections 16d, and the first drawing wiring sections 16c are arranged in parallel in the portions of the slits 18, 19 and between the slits 18 and 19.
As described above, in the conventional flexible wiring board 11, the first drawing wiring sections 16c are arranged in parallel in the portions of the slits 18, 19 and between the slits 18 and 19, with the result that the length in the vertical direction of the upper portion in FIG. 12 of the mounting region of the semiconductor chip 13 is rendered larger, leading to an increase in the entire length in the vertical direction. It follows that a first problem is generated that the flexible wiring board 11 is rendered bulky so as to increase the manufacturing cost.
What should also be noted is that, in the conventional semiconductor device, the inner leads 16b, 17b protrude into the device hole 15 of the film substrate 12. The protruding inner leads 16b and 17b tend to be deformed so as to bring about a short circuit between these inner leads 16b and 17b. Particularly, the pitch of the bump electrodes 6 is being made smaller and smaller in recent years in accordance with increase in the degree of integration of the semiconductor chip 13. As a result, the width of and the distance between the inner leads 16b and 17b are being diminished so as to cause the inner leads 16b, 17b to be highly likely to be deformed. A second important problem to be solved is how to cope with the defective bonding and occurrence of short circuit caused by the deformation.
A first object of the present invention is to solve the first problem given above so as to miniaturize the flexible wiring board that can be connected satisfactorily to an external circuit.
A second object of the present invention is to solve the second problem given above so as to prevent the connection terminal mounted to the film substrate from being deformed, thereby improving the reliability of the bonding with the bump electrode of the semiconductor chip.
For achieving the first object described above, there is provided a flexible wiring board in which the inclined wiring sections of a plurality of drawing wirings are rendered capable of being bent freely, and the inclined wiring region of the film having the inclined wiring sections mounted thereon are also rendered capable of being bent freely. As a result, it is unnecessary to arrange the wirings and the film covering the length of a plurality of slits for the bending and also covering the length of the parallel wiring section between these slits, which were formed in the past in the region other than the inclined wiring section, so as to achieve miniaturization of the flexible wiring board.
In order to achieve the bending degree that does not cause the wiring to be peeled off with a small stress, it is desirable for the film to have a thickness not smaller than 10 xcexcm and smaller than 40 xcexcm
In order to achieve the second object described above, a semiconductor chip is bonded to a film substrate by heating the surface of the semiconductor chip other than the surface on which a plurality of bump electrodes are mounted and by also heating the surface of the flexible wiring board other than the surface on which a plurality of connection terminals are mounted.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.